The invention relates to an electromagnetic switch with a housing provided with at least one magnet system, comprising a stator body of magnetic material with an exciter winding for the excitation of a magnetic field in the at least one magnet system, a swingably supported first arm which can be brought into motion under the influence of the excited magnetic field, at least one pair of contacts, and a leaf spring system acting upon the first arm.
1. Technical Field of the Invention
An electromagnetic switch of this type is known from U.S. Pat. Specification No. 2,883,488.
In practice, electromagnetically operable switches are used on a wide scale. Besides the application for the remote closing or opening of electric circuits, switches of this type are also used for the protection of electric circuits. Typical areas of application are zero voltage protection, forward and backward current protection, magnetic overload protection and earth leakage protection. To meet the specific switching functions of each of these areas of application, among others mechanical spring systems are used.
For example, spring systems are used, inter alia, for producing the force by means of which the contacts of the pair of contacts are held against each other (closed) or held apart (open) in order to meet specific conditions (thresholds) under which the contacts have to open or close, for producing a desired switching speed, etc. Leaf spring systems are advantageous in that a compact mechanical construction can be realized therewith.
However, the switch construction according to the said U.S. Patent Specification has insufficient facilities to meet the switching functions desired for the above-mentioned areas of application. Therefore, this known type of switch has a very limited field of application.
Other electromagnetic switches of this type known in practice generally having fairly complex spring systems with helical draw or compression springs, in conjunction with various switching arms and levers, for example such as described in French Patent Specification No. 866,592.
The various arms, levers and springs are usually fixed by one end to the housing or the chassis of the switch, so that the forces excerted on the pair of contacts also act on various points of the housing or the chassis, which are thereby usually loaded asymmetrically. In particular, in the case of switches for switching off short circuit currents in electrical installations, a high switching speed is required, which means that large powerful springs must be used. The fixing of the springs to the housing or the chassis will often necessitate a heavier structure and addition means, in order to obtain sufficient sturdiness, which means that the number of parts and the size of the switch generally increase as a higher switching speed is required.
A spring for achieving a particular switching function, for example producing a sufficiently high contact force, often has an adverse effect on another switching function such as, for example, the contact opening speed. An additional requirement is therefore generally that the spring action must be degressive. This means that the action of the spring system for producing one switching function must decrease very rapidly when another spring for another switching function goes into action and in some cases even must reverse in direction of action. In order to achieve such a degressive spring action, it is also necessary to have a relatively complex assembly of springs and levers which take up a large amount of space, such as disclosed in European Patent Application No. EP-A-127,784.
The object of the present invention is therefore to provide for an electromagnetic switch with a minimum of parts, and having a compact structure taking up little space, with which a large number of switching functions desired in practice, including short-circuit protection and overload protection (delayed/instantaneous switching) can be achieved.
2. Discussion of the Related Art
This is achieved according to the invention in that to actuate the pair of contacts a swingably supported second arm is provided, on each of said first and second arms one end of at least one leaf spring acts, the stator body having an oblong chamber accommodating a movable armature of magnetic material, said first and second arms extending on either side of the chamber at right angles to the lengthwise direction thereof in such a way that each of them can swing about their support point in the direction of the armature which is movable in the chamber between the arms, while the free end of the first arm is coupled to the armature, which has an operating element which acts on the second arm.
The placing of the armature between the first and second arm of the leaf spring system means that, by varying the swinging properties of one and/or the other arm, the movement of the armature as a result of the electromagnetic force exerted thereon can be set within wide limits. This means that the switching features of the switch can also be adapted with a great measure of freedom to the specific requirements set by a particular application. For example, with the first arm a desired threshold action against putting the armature into motion can be achieved, and with the second arm, independently of the action of the first arm, a desired force for holding the contacts against each other can be set.
Due to the mutual arrangement of the various moving parts of the switch according to the invention, the latter is both simple in design and compact in construction, and it can be dimensioned for a large number of specific purposes. Since the switch according to the invention also has a minimum of parts, the chances of failures either during fitting or in operation are much lower than in the case of switches of this type known in practice.
A preferred embodiment of the electromagnetic switch according to the invention is constructed in such a way that the stator body is approximately U-shaped in cross section, in which the chamber extends from one leg to the other, the leaf spring system is fixed to the stator body in such a way that the first and second arms each extend along a leg thereof, and the chamber and the legs are provided with passages through which the first arm is coupled to the armature and the operating element can act on the second arm, respectively, while the exciter winding is disposed around the chamber.
All moving switch parts are supported here by the stator body, so that no other connection points to the housing are necessary, other than the fixing points of the stator body itself. It will be clear that this is very attractive from the production point of view.
Another embodiment of the invention is characterized in that the armature is fixed to the first arm by means of a hinge connection and the operating element acting on the second arm is a pin connected to the armature, by means of which the second arm can be moved.
Yet another embodiment of the electromagnetic switch according to the invention is characterized in that the leaf spring system consists of a supporting frame with one leaf spring, each end of which acts respectively on the first and second arms situated in the frame aperture and supported by the supporting frame, the dimensions of said arms in the frame aperture being such that the leaf spring is tensioned, so that under the influence of the spring action of the leaf spring essentially a threshold action is obtained with the first arm against taking of the at least one pair of contacts into the one position and with the second arm essentially contact force is obtained for holding the pair of contacts in the other position.
Yet a further embodiment of the switch according to the invention is characterized in that the leaf spring system comprises a supporting frame with two leaf springs extending in such a way from an end at which they are supported by the supporting frame that the free ends of said leaf springs point in opposite directions, each free end acting on the first and second arms situated in the frame aperture and supported by the supporting frame, respectively, the dimensions of said arms in the frame aperture being such that the two leaf springs are tensioned so that with the spring action of the one leaf spring and the first arm influenced thereby essentially a threshold action is obtained against taking of the at least one pair of contacts into the one position, and with the spring action of the other leaf spring and the second arm influenced thereby essentially contact force for holding the at least one pair of contacts in the other position is obtained.
This embodiment has the advantage that the action of the first and second arms can be set in the optimum manner, independently of each other, since each of them is influenced by a separate leaf spring.
The swinging properties of an arm of the leaf spring system can be adapted in a relatively simple manner by selecting their support position and the action position of the at least one leaf spring thereon suitably relative to each other, this being of course partly dependent on the spring characteristics of the leaf spring itself. The leaf spring system is set forth in U.S. Ser. No. 291,257filed Dec. 28, 1988, and commonly owned herewith.
Since with a leaf spring system of this type such great forces can be exerted on the arms that not only sufficiently high switching speed, but also sufficiently high contact force can be achieved, the switch according to the invention can be used advantageously for switching off short circuit currents in, for example, electrical installations. An explosion chamber is often used in practice for extinguishing any discharge arc which may occur during separation from each other of the contacts of the pair of contacts of the switch. Yet another embodiment of the switch according to the invention, in which the at least one pair of contacts comprises a movable and a fixed contact, is for this purpose characterized in that the fixed contact is fixed to the inlet aperture of an explosion chamber accommodated in the housing, while the free end of the second arm contains the movable contact which can move in the inlet aperture of the explosion chamber.
The inlet of the explosion chamber in this construction is directly opposite the place of origin of the discharge arc, which promotes the extinguishing action thereof. A further improvement of the extinguishing action is obtained according to the invention through the fact that the explosion chamber is shaped so that it widens out from the inlet aperture, the wide part thereof containing the extinguishing means.
Yet another embodiment of the switch according to the invention which, in consequence of the high switching speed achievable therewith, in the order of magnitude of a fraction of the period duration of the alternating voltage to be switched off, can also be used advantageously as a current-limiting switch in electrical energy distribution plants, is characterized in that the switch has at least one current-limiting component, the connecting ends of which are connected to the respective contacts of the at least one pair of contacts. As a result of the very compact construction of the magnet system, this at least one current-limiting component can advantageously be housed in the housing of the switch. This current-limiting component is preferably a resistor, but can also be a reactance coil, or can consist of a combination of one or more resistors, reactance coils and/or capacitors.
Such a current-limiting switch is in practice always used in conjunction with a switch also accommodated upstream in the plant and has the advantage that the current of the electrical plant is only limited and is not interrupted. In the event of a short circuit and/or if there is a certain measure of overloading, the contacts of the current-limiting switch, which are normally closed under the effect of the spring action, are opened, so that the at least one current-limiting component is switched in series with the consumers connected to the plant. Through suitable dimensioning of the current-limiting component, the particular short-circuit or overload current is limited, after which this limited current is switched off by the said switch upstream housed in the plant. Since this further switch need not switch off the complete short-circuit current, its construction can be simpler, and it is thus less prone to failures than switches which have to be able to switch off a complete short circuit current.
If in accordance with a preferred embodiment of the switch according to the invention a connecting end of the exciter winding is connected to a contact of the at least one pair of contacts and the other connecting end of the exciter winding together with the other contact forms the connecting points of the switch, the limited current still continues to flow through the exciter winding. When a switch such as that constructed according to the invention is used, it is necessary for it to have a certain degree of hysteresis. This means that the current intensities at which the pair of contacts is moved from one position to the other and vice versa must be different. For example, when it is used as a current-limiting switch, the pair of contacts must open above a certain current intensity and must close at a current intensity lower than the limited current value.
In order to ensure that the switch goes into operation only at a particular intensity of the current in the exciter winding, a certain threshold action against taking the pair of contacts from one position to the other must be achieved. In the electromagnetic switch according to the invention, this can be achieved either by a suitable selection of the force applied by the at least one leaf spring on the first arm or by a suitable construction of the magnetic circuit of the magnet system.
In order to ensure that the pair of contacts generally returns again to a particular state at a current intensity which is lower than the current intensity necessary for taking it out of this position, an embodiment of the electromagnetic switch according to the invention is further characterized in that means are provided for limiting the travel of at least one of the arms in such a way that the total of forces exerted on the arms under the influence of the at least one leaf spring are always directed in the opposite direction to the forces which can be exerted on the arms under the influence of the armature.
Another embodiment of the electromagnetic switch according to the invention is to this end characterized in that a stop of magnetic material for limiting the travel of the armature is disposed in the chamber near the end situated opposite the second arm. With the same object, yet another embodiment of the invention is characterized in that another stop for limiting the travel of the second arm is provided in the housing.
Through the limitation of the travel of the armature, the travel of the first arm connected thereto and with the further stop the travel of the second arm can be limited in such a way that when the switch is energized, in other words, when the pair of contacts is moved from one position to the other under the influence of the generated electromagnetic force, the resultant of the forces exerted on the arms by the spring system is always such that it maintains a certain value which is opposite to the electromagnetic force of the magnet system. The stop accommodated in the chamber also has an effect on the magnetic action of the magnet system, which can be understood as follows.
In one position of the pair of contacts, for example if the contacts are resting against each other, there is an air gap of specific dimensions, and with a specific magnetic resistance, between the armature and the stop. In order to move the armature in the direction of the stop, the electromagnetic force generated by the current in the exciter winding will have to have a sufficiently high value to overcome this magnetic resistance, in addition to the threshold force exerted by the leaf spring system. When the armature has moved against the stop, so that the pair of contacts is open, the current through the exciter winding will be reduced by the electric arc occurring, but in particular in the embodiment with a current-limiting component. Since there is now no air gap between the armature and the stop in the chamber, this lower current is sufficient to hold the armature against the stop. If the current through the exciter winding now decreases further in such a way that the electromagnetic force becomes smaller than the force exerted by the spring system on the armature, the armature is moved under the influence of the spring action in the direction away from the stop, so that the pair of contacts closes again. It will be clear that the envisaged hysteresis and threshold action are determined both by the force exerted by the spring system on the armature and by the dimensions of the air gap.
In order to permit holding of the contact pair in a particular position, for example after switching, the second arm can according to the invention be locked in a known manner in such a way that the pair of contacts can be moved to the other position, for example simply by releasing the lock by hand.
The invention will now be explained with reference to two embodiments and the attached drawings.